Oxygen sensors are used in a variety of applications that require qualitative and quantitative analysis of gases. In automotive applications, the direct relationship between oxygen concentration in the exhaust gas and air to fuel ratio (A/F) of the fuel mixture supplied to the engine allows the oxygen sensor to provide oxygen concentration measurements for determination of optimum combustion conditions, maximization of fuel economy, and management of exhaust emissions.
One type of sensor uses an ionically conductive solid electrolyte between porous electrodes. For oxygen sensing, solid electrolyte sensors are used to measure oxygen activity differences between an unknown gas sample and a known gas sample. In the use of a sensor for automotive exhaust, the unknown gas is exhaust and the known gas, (i.e., reference gas), is usually atmospheric air because the oxygen content in air is relatively constant and readily accessible. This type of sensor is based on an electrochemical galvanic cell operating in a potentiometric mode to detect the relative amounts of oxygen present in an automobile engine""s exhaust. When opposite surfaces of this galvanic cell are exposed to different oxygen partial pressures, an electromotive force (xe2x80x9cemfxe2x80x9d) is developed between the electrodes according to the Nernst equation.
With the Nernst principle, chemical energy is converted into electromotive force. A gas sensor based upon this principle typically consists of an ionically conductive solid electrolyte material, a porous electrode with a porous protective overcoat exposed to exhaust gases (xe2x80x9cexhaust gas electrodexe2x80x9d), and a porous electrode exposed to a known gas"" partial pressure (xe2x80x9creference electrodexe2x80x9d). Sensors typically used in automotive applications use a yttrium stabilized zirconia based electrochemical galvanic cell with porous platinum electrodes, operating in potentiometric mode, to detect the relative amounts of a particular gas, such as oxygen for example, that is present in an automobile engine""s exhaust. Also, a typical sensor has a ceramic heater attached to help maintain the sensor""s ionic conductivity. When opposite surfaces of the galvanic cell are exposed to different oxygen partial pressures, an electromotive force is developed between the electrodes on the opposite surfaces of the zirconia wall, according to the Nernst equation:   E  =            (              RT                  4          ⁢          F                    )        ⁢    ln    ⁢          xe2x80x83        ⁢          (                        P                      O            2                    ref                          P                      O            2                              )      
Due to the large difference in oxygen partial pressure between fuel rich and fuel lean exhaust conditions, the electromotive force (emf) changes sharply at the stoichiometric point, giving rise to the characteristic switching behavior of these sensors. Consequently, these potentiometric oxygen sensors indicate qualitatively whether the engine is operating fuel-rich or fuel-lean, conditions without quantifying the actual air-to-fuel ratio of the exhaust mixture. Oxygen sensors measure all of the oxygen present in the exhaust to make the correct determination when the oxygen content (air) exactly equals the hydrocarbon content (fuel).
Oxygen sensors include a ceramic sensing element that is brought up to temperature by a heater. The heated sensing element is sensitive to water in the exhaust system. Traditionally, heated oxygen sensors have been subject to internal ceramic element cracking, especially in sensors disposed down stream from the catalytic converter, induced by condensate water in the exhaust. Water enters a vehicle""s exhaust system, including the gas sensor, due to condensation of combustion byproducts. As a result, the heated sensing element may be subject to ceramic cracking when the water contacts the hot element. The sudden impact of liquid water will cause severe thermal shock and cracking of the element, causing irreparable damage to the sensor. The problem has been sought to be rectified through special protective shields in the exhaust system or changes in the exhaust configuration.
Various vehicle and sensor shields and other techniques have been tried to limit this problem. These include special heater control circuits and modified sensor shields. Such remedies typically increase vehicle or sensor complexity, adding to cost of production. Vehicle shields have met with some success but, when incorrectly designed, have actually made the problem worse.
Traditionally, the typical oxygen sensor shield design use holes or openings of a louvered shape along the sides of the shield to direct exhaust gas to the sensing element. These designs, although not complex, do not provide sufficient protection against water impingement to the sensor. More complex designs include modifications of the traditional louvered shield, e.g., employing a double walled shield with holes in the side, or holes in both the side and tip end of the shield.
A second problem associated with liquid water impingement upon the sensor has also been detected. If the ceramic sensing element becomes wetted, the time to heat it to operating temperature is greatly extended. The ceramic element typically operates at minimum temperatures of 300xc2x0 C. to 400xc2x0 C., depending on sensor design and requirements, for satisfactory function. Tests have shown that water impingement increases the time to operation by five times or more. A shield design that prevents water impingement will therefore reduce heating time and increase efficiency.
The shield must protect the fragile ceramic sensing element from mechanical damage, exhaust impact, and other foreign materials and contaminants while allowing entrance of a sufficient amount of gas to promote productive exhaust sensing.
The drawbacks of the prior art are overcome by the gas sensor shield and method for manufacturing and use thereof. One embodiment of the gas sensor shield comprises: an elongated body comprising solid sides and a tip disposed across one end of said shield; and an opening disposed within said tip, wherein said opening comprises at least two elongated apertures.
One embodiment of the method for sensing gas comprises: exposing a gas sensor to a gas stream, said gas sensor comprising a sensing element in electrical communication with a wiring harness, wherein said wiring harness is disposed within an upper shell, and a shield disposed over at least a lower portion of said sensing element, wherein said shield comprises an elongated body comprising solid sides and a tip disposed across one end of said shield with an opening disposed within said tip, wherein said opening comprises at least two elongated apertures; passing gas through said opening to said sensing element; and determining the concentration of at least one component of said gas.
One embodiment of the method for manufacturing a gas sensor shield comprises: disposing a sensing element in electrical communication with a wiring harness, wherein said wiring harness is disposed within an upper shell; and disposing a shield over at least a lower portion of said sensing element, wherein said shield comprises an elongated body comprising solid sides and a tip disposed across one end of said shield, and an opening disposed within said tip, wherein said opening comprises at least two elongated apertures.
The above-described and other features will be appreciated and understood by those skilled in the art from the following detailed description, drawings, and claims.